ANKARA, Turkey -- Three Cabinet ministers resigned in Turkey on Wednesday, days after their sons were taken into custody in a sweeping corruption and bribery scandal that has targeted Prime Minister Recep Tayyip Erdogan's allies in one of the worst political crises of his more than 10 years in power.

Economy Minister Zafer Caglayan and Interior Minister Muammer Guler announced their resignations in statements carried by the state-run Anadolu Agency. Environment and Urban Planning Minister Erdogan Bayraktar gave his resignation from both the Cabinet and Parliament in a live interview with the private NTV television during which he also urged the prime minister to step down.

All three ministers denied any wrongdoing.

Caglayan's and Guler's sons, along with the chief executive officer of the state-run bank Halkbank, are among 24 people who have been arrested on bribery charges. Bayraktar's son, Abdullah Oguz, was detained as part of the investigation but later released from custody.

Media reports said the police have seized $4.5 million in cash that was stashed in shoe boxes in the home of the bank's CEO, while more than $1 million in cash was reportedly discovered in the home of Guler's son, Baris.

Erdogan has denounced the corruption inquiry as a plot by foreign and Turkish forces to thwart his country's growing prosperity and discredit his government ahead of local elections in March. Critics accuse Erdogan of becoming increasingly authoritarian, but his government has won three successive elections since 2002 on the strength of the relatively robust economy, a clean image and a promise to fight corruption.

The investigation is one of the biggest political challenges Erdogan has faced since his Islamic-based party narrowly escaped closure in 2008 for allegedly undermining Turkey's secular Constitution. This summer, the government also weathered a wave of anti-government protests sparked by a development project that would have engulfed a beloved Istanbul park.

Wednesday's resignations came as a surprise. Erdogan was expected to remove ministers implicated in the scandal quietly, through a Cabinet reshuffle that was planned even before the corruption allegations erupted. The planned reshuffle aimed to replace three other ministers who are standing for mayoral positions in the March local elections.

Turkish commentators believe the inquiry is fallout from an increasingly public feud and power struggle between Erdogan's government and an influential U.S.-based Muslim cleric, Fethullah Gulen, whose followers are believed to have a strong foothold within Turkey's police and judiciary. The two men, without naming each other, have been engaged in a war of words since the corruption investigation was launched on Dec. 17.

Gulen has denied being involved in the investigation. He left Turkey in 1999 after being accused by the then-secular government of plotting to establish an Islamic state. He was later cleared of that charge and allowed to return to his homeland, but he never has and is living in Pennsylvania.

In an address to his party's provincial leaders, Erdogan distanced himself from the ministers who resigned by emphasizing his party's record and determination to fight corruption. But he also repeated a claim that his government was the target of an international plot involving the media and "gangs" inside Turkey -- a tactic he has also used during the summer's anti-government protests to deflect criticism.

"There are media institutions, organizations and gangs in Turkey who think of others' interests rather than their own country's interests, and are working as spies in a treasonous manner," Erdogan said.

In a veiled attack on Gulen, Erdogan said: "They speak of the Quran and of Allah but are remembered for ... plots."

Last week, Erdogan threatened to expel ambassadors from Turkey after four pro-government newspapers accused U.S. Ambassador Francis J. Ricciardone of scheming against the government.

One newspaper, Sabah, pressed ahead with that allegation Wednesday by claiming that a U.S. diplomat had prodded a business group to join an "anti-government lobby."

That prompted a new rebuttal from the U.S. Embassy.

"Allegations targeting U.S. Embassy employees published in some media organs do not reflect the truth," the embassy said in a statement in Turkish. "To repeat once again: No one should endanger Turkey-U.S. relations through such intentional slander."

As he resigned Wednesday, Caglayan again questioned the legitimacy of the investigation, which is focusing on alleged illicit money transfers to Iran and alleged bribery for construction projects.

"It is clear that the operation is a dirty conspiracy against our government, our party and our country," he said in a brief statement. "I am leaving my position at the Economy Ministry to spoil this ugly plot, which has involved my colleagues and my son, and to allow for the truth to be exposed."

In a telephone interview with NTV television, Bayraktar also denied any wrongdoing, complained of being pressured into resigning by Erdogan and insisted "a great proportion" of construction projects that are allegedly under investigation were approved by the prime minister himself.

"I want to express my belief that the esteemed prime minister should also resign," Bayraktar said.

Guler, the interior minister, told reporters on Tuesday that he was the victim of a political plot and that there is nothing his family could not account for. He also said alleged wiretap recordings of a conversation with his son -- reportedly used as evidence by the police for the arrests -- were tampered with, and that the cash discovered in his son's house was money earned from the sale of a luxury villa.

The opposition had long called for Caglayan and Guler to resign, claiming their sons were taking bribes on behalf of their fathers, and insisted they should not remain in positions where they were able to influence the probe.

The government already has dismissed dozens of police officials either involved in the investigation or thought to be linked to Gulen. Journalists have been barred from entering police buildings, fuelling accusations from critics that the government is trying to impede the probe.

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